Method of casting reentrant angles



United States Patent O 3,395,749 METHOD F CASTING REENTRANT ANGLES George Blair, Tyvonnay, Witton-le-Wear, Bishop Auckland, Durham, England Filed Jan. 24, 1966, Ser. No. 522,558 8 Claims. (Cl. 164-122) This invention relates to the production of metal castings other than die castings, and is applicable generally to castings of all `ferrous and non-ferrous metals, but particularly steel, produced with the aid of moulds and cores of relatively low heat conductivity.

The freezing rate of .a casting in the mould is governed by the rate at which heat is extracted from the solidifying metal by the mould and core, if heat losses from surfaces exposed to atmosphere are ignored.

In the case of a casting having an internal corner (i.e. a tri-planar or similar reentrant portion) the rate of heat extraction by the mould is low, and the mould or core material is locally super-heated, creating a hot spot. This hot spot slows or hinders the solidiication of the metal in the corner and often gives rise to shrinkage which results in porosity or cracking extending into the metal from the inner surface thereof. Such a defect creates both a weakening and an impairment of the appearance of the casting, which is difficult and costly to repair, for example, by a welding or similar method, owing for one reason to the fact that such a relative reentrant corner is often relatively inaccesible.

A similar though less extensive eiect may be caused during solidication in a casting `at a reentrant angle (i.e. an angle at the junction of two surfaces).

It is of course common to make reentrant corners and angles with a radius, lbut this radius is usually made as small as possible, for example, not greater than the wall thickness of the metal. Increase of the radius offers a somewhat increased cooling surface, but still does not provide a homogeneously solid zone within the corner or angle, and so adds little extra sectional strength.

The invention has for an object to provide a method of and means for eliminating these disadvantages.

In a method of producing metal castings according to the invention, a reentrant corner or angle is cast with a projecting rib, whereby the local surface area for heat transfer during solidication is increased and the thinner section of the rib promotes dissipation of heat and progressive solidication without surface shrinkage.

In a reentrant corner the casting method may be performed to provide a unilateral rib extending partly in the angle between two of the three defining surfaces and partly diagonally over the third of said surfaces, Alternatively, in a reentrant corner, the casting method may be performed to provide a rib having three arms, each extending in the angle between two of the three defining surfaces. The ridge of the rib may have a radius of curvature greater than but preferably not more than twice the basic internal radius of the junction between any two of the surfaces. This produces a rib having two curved sides the radius of curvature of each of which is less than, e.g., :about half of, said radius of the junction.

In a reentrant angle, the casting may be performed to provide a rib which extends along the angle between the two defining surfaces, preferably proportioned as stated in the preceding paragraph. Evidently, where the angle is continuous, e.g. between a plane or curved surface and an intersecting cylindrical, oval, or like continuous curved surface, the rib will extend endlessly around the angle. When the angle terminates at a reentrant corner, the casting may be performed to provide a rib in such corner as hereinbefore recited, and another rib extending along the angle and terminating short of said corner. For example, the rib in the angle may be made to decrease in prole to zero substantially asymptotically with the continuation of the radius of curvature of the surface junction upon which the corner rib is based. l

The improved method of casting will be further described with reference to the accompanying drawings, wherein:

FIGS. 1 and 2 are fragmentary `sections of metal castings with reentrant angles, as heretofore produced.

FIG. 3 is a perspective View of a portion of a casting having `a reentrant corner, and provided with ribs according to the invention.

FIG. 4 is a perspective view of the like portion of a casting, showing another form of rib.

FIG. 5 is a section on the line V-V of FIG. 4, and

FIG. 6 is a section on the line VI-VI of FIG. 4.

In the drawings, FIG. 1 illustrates the occurrence, at a reentrant right angle between the two walls 1 and 2 of a casting, of a `shrinkage due to a low rate of heat extraction :by the material of a core or mould, which gives rise to a zone of porosity or cracking 3A extending into the metal from the inner surface of the angle. Owing to the more rapid cooling of th-e external angle 4, the shrinkage 3 tends to extend substantially equiangularly between the Walls 1 and 2, and to have a depth of approximately half the thickness of the metal on the equiangular or diagonal plane. The defect creates both a weakening and an impairment of the appearance of the casting, and may be -diicult to remedy by welding, especially if the interior of the angle is relatively inaccessible.

A similar and even more intensive effect may be caused during solidiiication in a casting at a reentrant corner formed .at the junction of three intersecting internal surfaces, for example, three mutually perpendicular surfaces.

Increase of the internal radius between the walls 1 and 2, as shown in FIG. 2, offers a somewhat increased surface area for cooling by transmission of heat to the mould or core, but still does not provide a homogeneous- 1y solid zone within the metal at the angle or corner. Shrinkage again occurs due to earlier cooling of an external zone 5, and in this case by reason of the larger bulk of the section the shrinkage 6y may be less intensive but extends over a larger internal area.

The improvement according to the invention consists in making castings with ribs in reentrant angles or corners,

variously provided as shown in FIGS. 3 and 4, and proportioned as illustrated by way of example in FIGS. 5 and 6.

By the provision of a -rib extending transversely to a section as aforesaid, the projecting and larger surface area of the rib and its relatively small cross-sectional area enable more rapid cooling in the rib and progressive solidifcation away from the ridge of the rib down its lateral surfaces and through a zone 7 (see FIG. 6) substantially circular in cross-section of which a portion approximating to one third is within the cross-section of the rib. A similar and substantially contiguous zone 8 (FIG. 6) is relatively rapidly cooled by heat dissipation from the exterior angle. Thus, if there is any tendency to local shrinkage due to less rapid cooling, this occurs at the line 9 of contact or approach of said two zones, that is, substantially at the neutral axis of the angle section. Shrinkage at this location has negligible effect on the local strength of the casting, which in any event is increased by the presence of the properly solidified rib having a greater radius of curvature R1 (FIG. 5) in its lateral aspect, than the basic radius R2. of the junction between the two walls 1 and 2.

A reentrant corner comprised as shown in FIG. 3 between three surfaces 1, 2 and 3 (which are shown as, but need not be, mutually perpendicular) may be cast with a J unilateral rib 10 extending partly in the angle between the surfaces 1 and 3 and partly diagonally over the surface 2. The rib is described as unilateral for the reason that it is symmetrical to a plane equiangular between the surfaces 1 and 3. The ridge 11 of the rib 10 has a radius R1, as shown in FIG. 5, substantially twice that of the basic internal radius R2 of the junction between the surfaces 1 and 2. Said radius R2 is somewhat greater than the wall thickness, and may be defined as equal to the distance, as shown in FIG. 6, from the intersection of the two surfaces 1 and 3 to the lines at which the concave sides 12 of the rib 10 merge into said surfaces. The radius R3 of said concave sides at its maximum depth is of course less than and may be up to about half of, said radius R2 of the junction.

Alternatively, as shown in FIG. 4, a reentrant corner between three surfaces 1, 2 and 3, may be cast with a rib 13 having three arms 14, 15, 16 each extending in the angle between two of the three surfaces defining the corner. Such rib may have proportions similar to those of the rib 10 as above described.

FIGS. 3 and 6 also show straight ribs 17, 18 cast in reentrant angles respectively between the pairs of surfaces 1, 2 and 2, 3. Each of these ribs may have a section similar in form and proportions to that of the rib 10 as shown in FIG. 6. Preferably, as also shown in FIG. 6, where such a rib 17 extends towards a reentrant corner provided with a rib 10, said rib 17 may decrease to zero substantially asymptotically with the continuation of the basic internal radius of curvature R2 of t'ne junction between said surfaces 2 and 3 upon which the corner rib 10 is based.

Evidently, in a casting having two surfaces intersecting at an angle, but without reentrant corners, a rib such as 17 may be cast to extend along said angle. In the case of a casting wherein a plane surface or a curved, i.e. substantially part-spherical or part-Ovoid, concave or convex, surface intersects a continuous curved surface, such as a cylindrical or oval prismatic surface, a continuous or endless rib may be cast in the reentrant angle between said two surfaces.

Means for performing the method of production of castings according to the invention comprise moulds and cores having external corners and/or angles in which are provided hollows to create in the casting ribs as hereinbefore dened and described.

The invention also extends to castings provided with ribs in reentrant corners and/or reentrant angles as hereinbefore defined and described.

What I claim and desire to secure by Letters Patent is:

1. A method of producing a metal castin-g having at CII least one reentrant angle, wherein a projecting rib is cast in said angle to provide increased local surface area for dissipation of heat from the metal and progressive solidiication without surface shrinkage.

2. The method according to claim 1 of producing a metal casting having three intersecting surfaces defining at least one reentrant corner, wherein said projection rib is unilateral and cast to extend partly in the angle between two of said three surfaces and partly diagonally over the third of said surfaces.

3. The method according to claim 1 of producing a metal casting having three intersecting surfaces defining at least one reentrant corner, wherein said projecting rib is cast with three arms, each arm extending in the angle between one of the couples of said three surfaces.

4. The method of producing a metal casting according to claim 2, wherein said rib is cast with a ridge having a radius of curvature, greater than and not more than twice the basic internal radius between any two of said three surfaces.

5. The method of producing a metal casting according to claim 3, wherein said rib is cast with the ridge of each of said three arms having at the corner a radius of curvature greaater than and not more than twice the basic internal radius between any two of said three surfaces.

6. The method according to claim 1 of producing a metal casting having two intersecting surfaces dening a reentrant angle, wherein said rib is cast to extend along the angle between said two defining surfaces.

7. The method according to claim 1 of producing a metal casting having a continuously curved surface intersected by another surface to present a continuous reentrant angle, wherein said rib is cast to extend endlessly around said angle.

8. The method of producing a metal casting according to claim 2, wherein at least one other rib is cast to extend along the angle between said third surface and one of the other two of said surfaces, said other rib decreasing to zero substantially asymptotically with the continuation of the basic internal radius of curvature of the surface junction of said corner.

References Cited UNITED STATES PATENTS 2,018,762 10/1935 Kauffman 164-125 2,075,039 3/1937 Kauffman 164-125 I. SPENCER OVERHOLSER, Primary Examiner.

E. MAR, Assistant Examiner. 

1. A METHOD OF PRODUCING A METAL CASTING HAVING AT LEAST ONE REENTRANT ANGLE, WHEREIN A PROJECTING RIB IS CAST IN SAID ANGLE TO PROVIDE INCREASED LOCAL SURFACE AREA FOR DISSIPATION OF HEAT FROM THE METAL AND PROGRESSIVE SOLIDIFICAITION WITHOUT SURFACE SHRINKAGE. 